A typical electronic image sensor includes a number of photosensitive picture elements (“pixels”) arranged in a two-dimensional array. The pixels accumulate charge carriers in response to light striking the pixels, and each pixel has a maximum amount of charge it can store. A phenomenon known as “blooming” occurs when the total number of charge carriers collected by a pixel exceeds the charge capacity for that pixel and the excess charge spills over into adjacent pixels. One known anti-blooming technique forms a lateral overflow drain (LOD) within a pixel to provide a means for draining the excess charge carriers from the pixel before the charge carriers spill into adjacent pixels.
FIGS. 1-3 depict a method of forming lateral overflow drain and channel stop regions in accordance with the prior art. Initially, as shown in FIG. 1, an insulating layer 100 is formed over a substrate or well 102. A nitride layer 104 is then formed over the insulating layer 100.
FIG. 2 illustrates mask layer 200 formed on nitride layer 104 and patterned to form an opening having a width W1. The portion of nitride layer 104 that is exposed in the opening is etched away. Nitride layer 104 is commonly over etched to accommodate for variations in the thickness of nitride layer 104. This overetching removes a portion 202 of insulating layer 100. A dopant is then implanted into substrate 102 (represented by arrows) to form channel stop 204. Channel stop 204 prevents charge carriers from spilling to horizontally adjacent pixels.
Mask layer 200 is then removed and another mask layer 300 is formed on the remaining nitride layer 104 and the exposed portion of insulating layer 100 (FIG. 3). Mask layer 300 is patterned to form an opening having a width W2, and the portion of nitride layer 104 exposed in the second opening is etched away. Again, nitride layer 104 is typically over etched to account for variations in the thickness of nitride layer 104, thereby removing another portion 302 of insulating layer 100. A dopant is then implanted into substrate 102 (represented by arrows) to form lateral overflow drain 304.
To ensure that all of nitride layer 104 that overlies the lateral overflow drain region is removed prior to the formation of the lateral overflow drain 302, W2 typically overlaps with W1, creating overlap area 306. When nitride layer 104 is etched as shown in FIGS. 2 and 3, the portion 308 of insulating layer 100 located in overlap area 306 is etched twice. This double-etching can remove portion 308 completely, thereby exposing the top surface of substrate 102 and allowing the substrate surface to be damaged during subsequent processing steps.
Exposing or damaging the top surface of substrate 102 can potentially result in contamination of substrate 102 and produce defects in the image sensor, such as, for example, cluster defects.